1. Field of the Invention
This invention related to a communication system. More particularly, the invention relates to a method and a system for compensation of audio signal components in a communication system, such as in a vehicle communication system.
2. Related Art
The use of various types of communication systems has been proliferating over the last few years. For example, communication systems are often incorporated in vehicles for different purposes. For example, it is possible to use speech recognition and voice commands of the driver for controlling predetermined electronic devices inside the vehicle. Additionally, telephone calls, such as in a conference call, are possible with two or more passengers within the vehicle. For example, a person sitting on a front seat and a person sitting on one of the back seats may talk to a third person on the other end of the line using a hands-free communication system inside the vehicle. Moreover, it is possible to use the communication system inside the vehicle for the communication of the different vehicle passengers to each other.
In vehicle communication systems, it may be difficult to hear speech audibly and clearly due to noise, other sounds in the vehicle or attenuation of the speech sound waves. Accordingly, the voice of one of the passengers may be detected using one or more microphones positioned in different locations in the vehicle. The signal detected by the microphone can be processed and then output using the loudspeakers of an audio module that is normally located in the vehicle. The signal emitted from the loudspeaker, however, is normally also detected by the microphone. To avoid acoustic feedback or other undesirable effects, the signals detected by the microphone have to be processed and such signal components have to be filtered out. Otherwise, undesirable feedback can occur in the system.
In vehicle audio systems, it has become possible to select different modes for reproducing an audio signal. By way of example, state of the art audio systems provide the possibility to either reproduce the sound in a stereo mode or in a surround sound mode. In the surround sound mode, additional time delays may be introduced in the different audio channels of the audio signal, so that the person sitting inside the vehicle has the impression of a surround sound audio system. When this audio system having a variable time delay in the different audio channels is used in connection with a vehicle communication system, the audio signal component emitted from the loudspeakers and then detected by the microphone should be removed to avoid unwanted echoes. In a surround sound mode, the signal amplifier introduces an additional time delay into the audio channel and the audio signal component detected by the microphone is delayed by the time delay introduced by the amplifier. Accordingly, an echo compensation unit for compensating acoustic echoes, by simulating the signal path from the loudspeaker to the microphone, should be able to simulate this signal path with a variable time delay. For the echo compensation of audio signal components from a signal detected by a microphone, a method with high computing power may be necessary. The required computer power mainly depends on the length of the filter of the echo compensation units. Thus generally, the greater the length of the filter, the more computer power needed.
Furthermore, it is possible that several microphones may be used for one seat to detect the speech signal of a passenger. Negative feedback can be avoided when adaptive filters are used for filtering out echoes and feedback signal components of the signals.
In addition to the communication signals output via the loudspeakers of the vehicle, audio modules reproducing audio signals, such as radio signals or signals from a music storage device such as a compact disc, are provided in the vehicles. These audio signals are output via the same loudspeakers, and they are also recorded by the microphones and again output via, the loudspeaker. If these audio signal components are not attenuated before being output as part of the signal detected by the microphone, the driver has the impression of an audio sound signal having reverberation.
The above-described vehicle communication systems are often incorporated into expensive and highly sophisticated vehicles having highly sophisticated audio components. When the audio module is used in connection with a vehicle communication system, the sound quality is deteriorated by the feedback of the audio signal components picked up by the microphone and again fed to the loudspeakers. To avoid this signal quality degradation, the audio signal may be disabled during the in-vehicle communication, or the audio signal components detected by the microphone may be filtered out in an effective way.
For compensation of audio signal components in a sound signal (also referred to as “echo compensation”), a filter may be used to simulate the audio signal components of a sound signal that has been emitted from the loudspeaker and then detected by the microphone. However, the audio signal component may be, for example, an audio signal of a classical piece of music, a pop piece of music, or perhaps an interview without music. For all these different kinds of music, the echo compensation may have to be carried out in a different way to be effective. The audio signal components of the audio signal can have, in the case of a stereo signal for example, completely independent audio channels. In other situations, such as, for example, in the case of speaking interviews or one speaking person, the two audio signal parts of the stereo signal may be completely linear, depending on the signals. The echo compensation for linearly dependent signals is a difficult task, as the adaptation algorithms for calculating filter coefficients generally do not have a well-defined solution. When the audio signal changes from a piece of music to a person speaking, it is desirable for the filters to be adapted to the new signal characteristics. This adaptation of the filter takes a certain amount of time and during this time unwanted echoes can occur.
Moreover, echo compensation filters seek to simulate the path of the sound wave in the vehicle by calculating the pulse response. The approximation step may not result in a non ambiguous and definite answer. Particularly in cases where the audio signal may be either a mono signal or a multi-channel signal, the different channels being completely linearly dependent from each other, a multi-channel stereo echo compensation filter may have the problem of finding the correct result. In other words, the stereo echo compensation filter may not be able to accurately simulate the interior of the vehicle through which the sound passed before it is detected by the microphone in a correct way.
Accordingly, a need exists to effectively cope with the different situations that can occur in the compensation of audio signal components in an echo compensation unit, and generally for an improved system and method for compensation of audio signal components in a vehicle communication system. A need further exists to reduce the length of filters while maintaining a length sufficient to allow the echo compensation unit to be able to simulate the signal path of a stereo signal or of a signal in a surround sound mode. Yet a further need exists to effectively cope with the different situations that can occur in the compensation of audio signal components in an echo compensation unit.